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Full text of "Text Book Of Mechanical Engineering"

Mixtures.                                 41

has brought his ladle under the mouth of the cupola ; the other
rod has a flat end, with a lump of clay adhering. As soon as the
ladle is full, he applies the clay to the mouth in order to stop the
flow. To tap with safety, he should stand on one side of the
trough and use his tapping rod obliquely, while on stopping again,
he should cut off the stream from the top side.

Mixtures of Iron.—This is another art which nothing but
observation and practical experience can reduce to a nicety,
different mixtures being used for the same purpose by different
founders; indeed, the success of certain firms depends in a great
measure on the mixtures used. Still, a good idea can be given
of what is required for each purpose.

The varieties of pig iron having been already stated, we will
now consider each separately.

No. i is the weakest but most fluid of all the pigs, and may
be used by itself for ornamental castings on account of the ease
with which it fills the corners of the mould, but it is usual to mix
it with ' scrap J to increase its hardness, ultimate strength, and
closeness of grain.

No. 2 is finer in grain and stronger than No. i, and is used
wherever some strength is required with great fluidity.

No. 3 combines the greatest degree of strength consistent
with fluidity, and is therefore most extensively used, and in great
favour with founders.

No. 4 is the strongest pig for foundry use (the remaining
numbers, 5 and upward, being only required for conversion into
wrought iron), and is therefore used for heavy castings requiring
strength, such as girders, columns, bed plates, &c.

For strong castings two-thirds of No. 4 may also be used with
one-third of No. i.

Scrap is the name given to the broken up parts of old
castings, which of course may be divided into good and bad
scrap. Some founders place great reliance on it, using nothing
but scrap with an admixture of No. i, say two-thirds of scrap to
one-third of No. i, while others prefer using an iron like No. 3,
mixing with it only a little scrap to strengthen it, and so produce
a harder, close-grained casting. It is also a good plan to mix
iron from different blasts.